1. Field of the Invention
The present invention relates to magnetic resonance imaging (MRI) scanners, which are used for non-invasive imaging of the internal organs of a patient for medical diagnostic purposes, and more particularly to a radio frequency amplifier for use with an MRI scanner.
2. Description of the Related Art
Magnetic Resonance Imaging (MRI) is a well-known procedure based on nuclear magnetic resonance (NMR) principles for obtaining detailed, two and three dimensional images of patients. MRI is well suited for the imaging of soft tissues and is primarily used for diagnosing internal injuries.
Typical MRI systems include a magnet capable of producing an intense, homogenous magnetic field around a patient or portion of the patient; a radio frequency (RF) transmitter and receiver system, including a transmit/receiver RF coil also surrounds a portion of the patient; a magnetic gradient system localizes a portion of the patient; and a computer processing/imaging system, which receives the demodulated signals from the receiver system and processes the signals into interpretable data, such as visual images.
The superconducting magnet is used in conjunction with a magnetic gradient coil assembly, which is sequentially pulsed to create a sequence of controlled gradients in the main magnetic field during an MRI data gathering sequence. The superconducting magnet and the magnetic gradient coil assembly include the radio frequency coil on an inner circumferential side of the magnetic gradient coil assembly. The controlled sequential gradients are effectuated throughout a patient imaging volume (patient bore) which is coupled to at least one MRI RF coil or antenna. The RF coils and an RF shield are typically located between the magnetic gradient coil assembly and the patient bore.
As a part of a typical MRI, RF signals of suitable frequencies are transmitted into the patient bore. Nuclear magnetic resonance responsive RF signals are received from the patient via the RF coils. Information encoded within the frequency and phase parameters of the received RF signals, by the use of an RF circuit, is processed to form visual images. These visual images represent the distribution of NMR nuclei within a cross-section or volume of the patient, within the patient bore.
In modern MRI, the demand for high spatial and temporal resolution necessitates the use of high static magnetic field. Active electric coils are used to drive spatial gradients into the static magnetic field. Enhanced imaging sequences typically demand high amplitude gradient fields, rapid field transitions, and large duty cycles in order to improve resolution and scan time. Unfortunately, these properties also increase the power dissipation and thus cause higher temperatures in the scanner.
With reference to FIG. 1, the radio frequency transmit signal is produced by an RF oscillator 100 and fed to an RF amplifier 102 that drives a transmit coil 104 located around the patient. The RF oscillator 100 and amplifier 102 usually are located in an equipment cabinet in another room from the patient scanner and connected to the transmit coil 104 by a coaxial cable 106 that often is about ten meters long. Typically, the RF amplifier 102 of choice has been a Class-AB analog amplifier. The class of an analog amplifier defines what proportion of the input signal cycle is used to actually switch on the amplifying device. With a Class-AB amplifier more than 50%, but less than 100%, of the signal cycle that is used to switch on the amplifier. Unfortunately, these amplifiers are not very efficient and produce a significant amount of heat. The efficiency of a power amplifier is defined as the ratio of output power and input power expressed as a percentage and due to heat production.
Linearity is another important characteristic desired for the RF amplifier. A Class-A amplifier has very good linearity, but this type of amplifier is less than 10% efficient, therefore its power consumption is relatively high. Class-B amplifiers have somewhat worse linearity but are only on half of the time, and rely on the flywheel effect of the resonator to come around. This type of amplifier is, therefore, much more efficient than class A. The disadvantage is linearity, which is decreased to a level that is less desirable. Class-AB amplifiers are in between Classes A and B. Class-C amplifiers are more efficient than Class-B, but have worse linearity. They are on less than half of the signal cycle. Any amplifier that greatly relies on the flywheel effect is required to be close to or right on the resonator for maximum effect.
Recently, a different kind of amplifier, known as a switching amplifier, has been developed. A particularly useful switching amplifier is called a Class-E amplifier. Switching amplifiers have relatively high power efficiency due to the fact that perfect switching operation does not dissipate power. An ideal switch has zero impedance when closed and infinite impedance when open, implying that there is zero voltage across the switch when it conducts current (on state) and a non-zero voltage across it in the non-conductive state (off state). Consequently, the product of voltage and current (power loss) is zero at any time. Therefore, a Class-E amplifier has a theoretical efficiency of 100%, assuming ideal switching.
Although those advantages of Class-E amplifiers could be beneficial for an RF amplifier in a magnetic resonance imaging system, their high non-linearity make conventional amplifiers of this type undesirable for MRI applications.